using two more CNAME RRs

[johnyb98]

Hello everybody!

Please, I am trying to understand the following CNAME RR settings issue.
Let's say that on a registrar I have root domain e.g. test.com

 

On the CNAME RRs, I understand when I build CNAME record:

 

CNAME                              host: www.test.com                                 points to: test.com

 

This is for, when someone types on browser's address bar www.test.com , it is forwarded to test.com . I have no problem on understanding with this.
The one that I do not understand is the following:

 

For a mail server build, for domain test.com , I see that some domain admins also add CNAME records like:

 

CNAME                              host: mail.test.com                      points to: test.com
CNAME                              host: www.mail.test.com             points to: test.com

 

How they work? Why are they entered?

 

Can you please help me with these enters?

 

Thank you for your time!

[NelizMastr]

Seems useless to me. I never do that anyway, there's no really valid point.

 

I usually point mail/webmail.domain.com directly to the mailserver, and add an autodiscover record as well (as I only really ever setup Microsoft Exchange).

The CNAMEs your describing are mainly to catch users that are typing in the wrong URL.

[jasonvp]

On 6/16/2021 at 5:26 AM, johnyb98 said:

How they work? Why are they entered?

 

Can you please help me with these enters?

This smells like a quiz or homework question, but I'll play along.

 

As it turns out, this is quite common in DNS.  If you have an A-record for domain test.com, and that A-record is the same IP address(es) as mail.test.com and www.mail.test.com, it allows you to aim those two aliases (CNAMEs) at it.  If you ever have to change the IP address(es) for test.com, you only have to make one edit versus three.

 

[Alex Atkin UK]

All these things are just ease of use.  I never use CNAMES as they force a double-lookup vs using an A record, but if you are a business with a lot of sub-domains then it could save you the hassle of changing all the entries if you move hosts.

 

I believe the only reason mail. is a thing is again ease of use if you are a business.  That way if your traffic gets so high you need to move your mail server to a dedicated host you don't have to tell everyone to change the domain name in their clients.

 

From an efficiency point of view, you'd never use CNAMEs.

[jasonvp]

2 hours ago, Alex Atkin UK said:

From an efficiency point of view, you'd never use CNAMEs.

 

You're basically micro-optimizing here, and in reality it's not going to make a lick of difference.  Been there, done that, have over 30 years of running DNS.

 

[Alex Atkin UK]

On 6/18/2021 at 7:53 PM, jasonvp said:

 

You're basically micro-optimizing here, and in reality it's not going to make a lick of difference.  Been there, done that, have over 30 years of running DNS.

 

I guess it WAS more important "back in the day" when a DNS lookup could be costly because we were all stuck with crappy ISP DNS, back before Google, Cloudflare, etc were a thing.